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My world is science-based with an earth-like biology. The dragons fly when they're young and then grow too heavy and can only glide if at all. They breathe fire by expelling gas and igniting it. They are carnivores. See How could dragons be explained without magic? for more.

Knowing that dragons look like big lizards, we could assume they are cold blooded. But dinosaurs were neither cold nor warm blooded. And finally, they do breathe out fire (probably from exhaling a gas or a combination of gases that then ignite) so they must be able to withstand some relatively high temperatures...

So what makes the most sense? Hot, cold, or neither?

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    $\begingroup$ While this is an interesting question is's also impossible to answer. As mythical beasts dragons could be either or something else entirely. The question needs to be much more specific. $\endgroup$ – Tim B Sep 22 '14 at 12:46
  • $\begingroup$ I believe with scientific arguments, the "most likely" case can be determined. @WayneWerner's answer is a good example: cold blooded requires less food intake and fits with the ability to produce fire. $\endgroup$ – Sheraff Sep 22 '14 at 12:50
  • $\begingroup$ General reference questions tend to get really vague answers, while good answers to specific questions give enough information that they can be generalised to other situations. Without a specific context or goal, this is basically an exercise in who can make the most convincing-sounding justification. The one answer given so far says exactly that: without a specific context for the dragon (habits, environment, behaviour, diet, etc.) we can invent any biology to justify any answer. $\endgroup$ – BESW Sep 22 '14 at 12:52
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    $\begingroup$ @FlorianPellet The problem is that we are discussing hypothetical creatures in a hypothetical environment under hypothetical constraints. The result is an incredibly broad discussion with no real possible resolution. $\endgroup$ – Tim B Sep 22 '14 at 12:59
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    $\begingroup$ As someone with a degree in physiology and zoology amongst other subjects, I disagree that this question is too broad. The problem is well defined, with a narrow range of scientifically plausible solutions, which I have detailed in my answer. Are there any others here prepared to claim similar real-world expertise who want to disagree with me? $\endgroup$ – Monty Wild Sep 22 '14 at 16:11
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Dragons as stated with the ability to fly when young and with the ability to produce flammable gas are leading a quite high-energy lifestyle, which would require a large energy input. As they are carnivores, this dictates a certain lifestyle.

Cold-blooded carnivores are ambush predators. They excel at sitting still for long periods waiting for something unwary to happen by that they can catch and eat. Since they don't generate their own body heat, their baseline metabolic requirements are quite low.

Warm blooded carnivores have a higher energy requirement simply due to the fact that they must maintain their body temperature, typically above the ambient air temperature. This means that they must be more proactive in finding food, to the point where they actively hunt for their prey.

For a creature that must expend large amounts of energy flying and generating flammable gas, it seems unlikely that a cold-blooded metabolism could provide the capability to obtain sufficient energy input for their lifestyle.

This means that dragons must be - to some degree - able to regulate their own body temperature independent of the environment so as to be able to hunt in weather conditions that would not favor a cold-blooded animal.

All animals, whether homoeothermic or poikilothermic, require more energy input per unit mass when small than when they are large, and the gradients are identical, i.e. by increasing mass a certain amount, the energy requirement per unit mass goes down proportionally, regardless of whether the creature is homoeothermic or poikilothermic.

In addition, marsupials have lower energy requirements per unit mass than placental mammals of the same mass, but not as little as a poikilothermic creature, so it is not beyond the bounds of possibility for dragons (or dinosaurs) to be homeotherms and still have a lower energy requirement than a placental mammal.

Other strategies employed by mammals to reduce energy expenditure are sleep (practiced by cats) and hibernation (practiced by bears and rodents). The ability to spend long periods sleeping or hibernation would fit well with some popular representations of dragons.

In conclusion, I would suggest that dragons are most likely to be lower-metabolism homeotherms (i.e. warm blooded) than placental mammals with a propensity to long periods of sleep and hibernation.

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  • $\begingroup$ Is that correct that cold blooded animals "don't generate their own body heat"? I thought "cold blooded" means they cannot maintain constant body temperatures. $\endgroup$ – enkryptor May 25 '16 at 6:38
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    $\begingroup$ @enkryptor, poikilotherms "don't generate their own body heat" in the context that they cannot increase their own body temperature relative to that of the environment without engaging in physical activity. This physical activity must be something like running, as poikilotherms cannot shiver. $\endgroup$ – Monty Wild May 25 '16 at 6:52
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They could be either, especially if you choose different biology to explain their existence.

Dragons are often found in a variety of temperatures, from high above the surface of the earth, to arid deserts. If they were warm blooded, they could easily withstand these temperature differences, at the cost of eating tons of food for a large dragon, especially to keep warm.

However, if they were cold-blooded then they would naturally enjoy the heat, but if they had some type fuel source it's perfectly reasonable to suggest they can use that heat source to regulate their temperature. Also apparently cold-blooded creatures need to eat much less.

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I think an interesting option could also be that dragons can switch between cold-blooded and warm-blooded metabolism. For example, they could be warm-blooded when young (providing them the needed energy to fly), but then convert to cold-blooded (or in-between) when they get old (so they can grow without having to grow their food intake at the same rate; at old age, they are no more able to fly anyway, according to your assumption).

The conversion might be a continuous process (they start warm-blooded, but their metabolism gradually goes down as they grow), or it could be a specific transition phase at a certain age (not too different from puberty or menopause for humans, except of course humans don't change their warm-bloodedness in that process). Such a transition phase would then also go with specific noticeable behaviour changes.

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    $\begingroup$ is that something that exists in nature? $\endgroup$ – Sheraff Oct 8 '14 at 8:12
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    $\begingroup$ I'm not aware of any example, but I don't see a reason why it could not exist in principle. $\endgroup$ – celtschk Oct 8 '14 at 8:13
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    $\begingroup$ @Sheraff: According to this Wikipedia article bats basically switch between warm- and cold-blooded, but not as function of age, but as part of their activity/sleep cycle. $\endgroup$ – celtschk Oct 8 '14 at 13:07
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    $\begingroup$ Dragons are often found slumbering for years, which could be interpreted as swapping to cold-blooded for hibernation. $\endgroup$ – Tigt Oct 13 '14 at 23:21
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Some dinosaurs are certainly warm blooded, as many scientists now consider birds to be dinosaurs. But you might want to put feathers on the dragons, as these both are useful for heat conservation AND flight. Or you could make them some sort of fire breathing Pterosaur.

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Personally, I would go with warm blooded since higher metabolism would allow for more active, hunting dragons which could help justify their large size. Also, a higher metabolism could support more brain-power (if that's something you care about).

I can see how you could make cold blooded dragons work though. They would likely be opportunistic hunters that hibernate through cold temperatures. The catch here is that large dragons will need creative hiding methods. They have to surprise their prey, but they can also go months without food, so... I guess you could have dragons covered in fauna disguised as small hills?

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You could go either way.

  1. There is some evidence that some dinosaurs were warm-blooded, as there is plant-based evidence they lived in cooler climates.
    1. Here's one theory based on their growth rate.
    2. Here's a new term 'mesothermic' to indicate some dinosaurs had a physiology between warm- and cold-blooded.
  2. There have been thermal cameras pointing at bees showing they generate their own heat. I have personally seen bumble bees buzzing around when it's 50F outside, but only in the past few years. This is a first for me.
    1. Insect thermoregulation.
    2. Study: Bees shiver to produce heat in winter clusters.
  3. Vertebrate carnivores have a tendency to be warm-blooded, because they need the extra calories meat provides to generate their own heat.
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  • $\begingroup$ Regarding point 3: The argument seems backwards; Carnivores do not become warm-blooded because they need energy from meat to generate body heat. One might say that warm-blooded animals become carnivores for this reason, but this does not seem to be the case for extant animals. To the contrary, warm-blooded vertebrate non-carnivores (rodents, ruminants, seed-eating birds, etc.) are very common, as are cold-blooded vertebrate carnivores (snakes, many lizards, amphibians, most carnivorous fish). $\endgroup$ – brendan Mar 2 '18 at 14:08
  • $\begingroup$ In fact, among terrestrial vertebrates, I would say that the trend is the opposite: almost all terrestrial vertebrate herbivores are warm-blooded. $\endgroup$ – brendan Mar 2 '18 at 14:08

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